Are the low frequencies modulated with a 3.3Mhz carrier? I believe that low frequency audio is not very useful!

Low frequencies are effective for many uses. Targetting viruses and bacteria is always best done with high frequencies, if the frequency is known. A carrier cannot be directly demodulated by the body and does not serve much direct function apart from producing sidebands.

The information carried by the low frequency passes straight through without being used. Carriers (such as 3.3 MHz) are used by plasma systems because they are designed to resonate at a fixed frequency. At resonance the voltage is very high. This high voltage ionises the gas in a plasma tube.